Trigonometry/e to the i Pi

Take the formula

$e^{xi}=\cos (x)+i\sin (x)$ .

If $x=\pi$ radians, $\cos (x)=-1$ and $\sin (x)=0$ so the formula becomes

$e^{\pi i}=-1$

or

$e^{\pi i}+1=0$

a formula relating five key numbers in mathematics.